1. Field of the Invention
The present invention relates to a picture processing apparatus and a processing method thereof for enlarging/reducing a picture with any ratio, the apparatus and method being applicable for a special effect generating unit used in a broadcast station, a television receiver, or a video tape recorder.
2. Description of the Prior Art
As an interpolating method for enlarging/reducing a picture with a desired ratio free of deterioration of the resultant picture, linearly interpolating method is known. In the linearly interpolating method, the position of any point (pixel) of an enlarged/reduced picture (hereinafter referred to as converted picture) corresponding to an original picture that has not been enlarged/reduced (hereinafter referred to as original picture) is obtained. Corresponding to gradation values of four adjacent pixels of the obtained position, the gradation value of the desired position of the converted picture is obtained.
For example, as shown in FIG. 8, it is assumed that an original picture 100 is enlarged with any magnification k (for example, 1.3 times) and thereby a converted picture 101 is formed. First, a point a of the original picture 100 corresponding to a point A of the converted picture 101 is obtained. The coordinates of the point a can be obtained by dividing the coordinate values of the point A by the magnification k. Assuming that the coordinates of the point A are A(5, 3), the coordinates of the point a are obtained as a(5/1.3, 3/1.3)=a(3.8462, 2.3077).
As shown in FIG. 9, the point a is present in an area surrounded by four adjacent points represented by a.sub.00 (3, 2), a.sub.10 (4, 2), a.sub.01 (3, 3), and a.sub.11 (4, 3). The gradation value of the point a can be obtained by the sum of products of the gradation values (pixel data) of the pixels at these four points and the internally divided ratio of p and q in the area surrounded by the four adjacent points of the point a. In other words, the gradation value of the point a can be obtained by the following formula (1). EQU A=(1-p) (1-q).cndot.a.sub.00 +p.cndot.(1-q).cndot.a.sub.10 +(1-p)q.cndot.a.sub.01 +pq.cndot.a.sub.11 (1)
The gradation value of the point a is equal to the gradation value of the point A on the converted coordinates. Thus, by performing calculations of the formula (1) for all pixels of the converted picture, pixels are interpolated. The internally divided ratio of p and q is equivalent to an interpolation coefficient in the horizontal direction and an interpolation coefficient in the vertical direction, respectively. In this process, a point on the converted coordinates is mapped to a point on the original picture so as to prevent a pixel from being lost.
FIG. 10 shows an example of the structure that accomplishes such a process. Picture data of an original picture is received from an input terminal 110. The received original picture data is supplied to low pass filters 111 and 112 that perform a low pass process in the horizontal direction and a low pass process in the vertical direction, respectively. The resultant data is written to field memories 113a, 113b, 113c, and 113d. The same original picture data is written to the field memories 113a, 113b, 113c, and 113d.
The position of the point a of the original picture corresponding to any pixel A of the converted picture is obtained in a predetermined method. Corresponding to the position of the point a, coordinates a.sub.00, a.sub.10, a.sub.01, and a.sub.11 of four pixels adjacent to the point a are obtained. Corresponding to these coordinates a.sub.00, a.sub.10, a.sub.01, and a.sub.11, addresses for which gradation values (pixel data) of pixels corresponding to the coordinates a.sub.00, a.sub.10, a.sub.01, and a.sub.11 are read from the field memories 113a, 113b, 113c, and 113d are generated, respectively. The generated addresses are supplied to the memories 113a, 113b, 113c, and 113d (not shown).
The gradation values of pixels corresponding to the coordinates a.sub.00, a.sub.10, a.sub.01, and a.sub.11 are read from the field memories 113a, 113b, 113c, and 113d, respectively. The resultant gradation values are supplied to a vertically/horizontally interpolating circuit 114. The vertically/horizontally interpolating circuit 114 is composed of a plurality of multiplying devices and a plurality of adding devices so as to accomplish the calculation of the formula (1). The vertically/horizontally interpolating circuit 114 calculates the formula (1) corresponding to the gradation values of the pixels of the coordinates a.sub.00, a.sub.10, a.sub.01, and a.sub.11 so as to interpolate the gradation value of the pixel A of the converted picture. The obtained gradation value is supplied to an output terminal 115.
By performing such an interpolating process for all pixels of the converted picture, a converted picture of which the original picture is enlarged for example 1.3 times can be obtained.
In the conventional method, to obtain the gradation value of the pixel A of the converted picture, gradation values of four points of the original picture are required on real time basis. The coordinate values of the four points are varied corresponding to the size of the converted picture (namely, the reduction/enlargement ratio of the converted picture to the original picture). Thus, the four field memories 113a, 113b, 113c, and 113d to which the same original picture data is written and that is randomly accessed are required. Thus, the cost of the final product becomes high.
Moreover, in the above-described formula (1), since a multiplying process should be performed eight times, eight multiplying devices are required to accomplish the formula (1). These multiplying devices cause the cost of the final product to rise.
To reduce the cost of the final product, conventionally, most adjacent pixel interpolating method has been used. In this method, a pixel most adjacent to the position of a point a of an original picture is used as a pixel A of a converted picture. Since the accuracy of the interpolating process in this method is low, a high picture quality cannot be obtained.
Moreover, as described above, in the structure of the prior art reference, a linear interpolating circuit for reducing/enlarging a picture should be disposed upstream of the field memories.